/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/MoviePipelineSurfaceReader.h
Maps the ReadbackTexture to the CPU (which should have been resolved to before this point) and copies the data over.
void CopyReadbackTexture_RenderThread ( [TUniqueFunction](API\Runtime\Core\Templates\TUniqueFunction)< void([TUniquePtr](API\Runtime\Core\Templates\TUniquePtr)< [FImagePixelData](API\Runtime\ImageWriteQueue\FImagePixelData) > &&)> && InFunctionCallback, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FImagePixelDataPayload](API\Plugins\MovieRenderPipelineCore\FImagePixelDataPayload), ESPMode::ThreadSafe > InFramePayload )